1. Field of the Invention
The present invention relates to a method for forming bumps on electrode pads provided on a substrate, to an electronic component on which bumps are formed, and to a solder paste.
2. Description of the Related Art
There has been a growing need for higher mounting density with electronic components in recent years, and bare chip mounting methods have been attracting attention. There are two types of bare chip mounting method: a face-up method involving wire bonding, and a face-down method featuring metal bumps. Face-down mounting is becoming more and more prevalent today. A benefit of connecting with metal bumps by face-down method is the lower resistance of the connection. On the other hand, numerous demands are imposed on this method, such as lower cost, ensuring a precise bump height in order to achieve stable connection reliability, and forming bumps at a fine pitch corresponding to the electrode pads of a semiconductor chip.
Plating and vapor deposition are just two conventional bump formation methods. These bump formation methods require a tremendous equipment investment, and make it difficult to control bump height and metal composition, among other problems. In view of this, engineers have been taking a closer look at printing, which allows a metal paste to be supplied at low cost.
One type of printing method makes use of a metal mask. In addition, as disclosed in JP-A-7-273439 and JP-A-11-340270 and elsewhere, there is also a method that utilizes a resin mask. When a metal mask is used, one in which openings have been formed corresponding to the locations where the electrode pads are formed is placed over a substrate. When a resin mask is used, a resin layer is formed over a substrate, after which the portions corresponding to the electrode pads are removed to form openings. The two methods are similar in that after this, a squeegee is used to push a solder paste applied over the mask into the openings and thereby form bumps. When a metal mask is used, it is removed after the openings have been filled with the solder paste, but when a resin mask is used, it is removed as needed after the bumps have been formed.
However, if a large proportion of the solder powder that makes up the solder paste has a large particle diameter (such as an average particle diameter of 30 to 40 xcexcm), there tends to be variance in the size of the bumps that are formed. Causes of this include the fact that some of the solder powder that has filled the openings is wiped away when the squeegee is moved back and forth over the mask, and that when the metal mask is removed after the openings have been filled with the solder paste, the solder paste clinging to the inner walls of the openings ends up being taken away with the mask.
To avoid this problem it is necessary to use a solder powder with a small proportion of particles whose diameter is large. For instance, it is good to use a solder powder with a large proportion of particles whose diameter is no more than ⅓ the thickness of the mask (when the thickness of masks commonly in use is considered, this is substantially a particle diameter of 15 xcexcm or less).
Meanwhile, methods for producing a solder powder include disc atomizing and gas atomizing. With these methods it is difficult to stably produce a powder with a small particle diameter. Accordingly, the current approach is to produce a powder having a particle size distribution within a certain range, and then separate and collect the fines. However, not only does separating out the fines require considerable labor, it is also difficult to collect a large quantity of fines. For instance, with existing technology a solder powder of 20 xcexcm or less only accounts for about 20% of the total powder, which is also disadvantageous in terms of cost. Also, because a fine powder with a small particle size has a larger specific surface area and is therefore oxidized more readily, the solder paste made up of this solder powder has a shorter life.
The bump formation method provided by the first aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of electrode pads, comprising the steps of providing a mask having a plurality of openings corresponding to the plurality of electrode pads, filling each of the openings with a solder paste, and heat treating the solder paste, wherein the solder paste contains solder powder and a flux vehicle, and the solder powder contains no more than 10 wt % particles whose diameter is greater than the thickness of the mask and no more than 1.5 times this thickness.
Unless otherwise specified, the term xe2x80x9csubstratexe2x80x9d as used in the present invention includes all substrates on which electrode pads are formed, which of course includes circuit substrates and silicon wafers, but also includes semiconductor chips and so forth. When an opening is not circular, xe2x80x9copen diameterxe2x80x9d refers to the diameter of a circle having a surface area equivalent to the surface area of the opening.
The solder paste used in this bump formation method must have a small proportion of solder powder with a relatively large particle diameter as compared to the thickness of the mask. This reduces the danger that the solder paste filling the openings will be wiped away when the mask is coated with the solder paste and a squeegee is then moved back and forth over the mask in an effort to pack the insides of the openings with the solder paste. Also, when a metal mask is used, there will be less danger that the solder paste clinging to the inner walls of the openings will be taken away with the metal mask when the mask is removed after the openings have been filled with the solder paste. Accordingly, there will be less variance in the bumps if they are formed by the above method.
The smaller is the quantity of solder powder within the above-mentioned particle diameter range, the more pronounced this effect will be, and the ideal proportion for such solder powder is therefore 0 wt %. For the above effect to be realized even better, it is preferable to use no more than 10 wt % solder powder having a particle diameter of 40% or more of the open diameter of the openings.
In a preferred embodiment, the solder powder contains at least 30 wt %, and preferably at least 50 wt %, particles whose diameter is 40 to 100% of the mask thickness.
This solder paste has a larger proportion of solder powder of suitable particle size as compared to the mask thickness, and a smaller proportion of solder powder of relatively small particle size. If the thickness of the mask is about 50 to 100 xcexcm, for example, then the proportion of solder powder having a particle diameter of 20 xcexcm or less is small. As discussed above, it used to be that preparing a solder powder having a particle diameter of 20 xcexcm or less not only was labor intensive, but also produced a low yield and was expensive, but if the proportion of solder powder with such a particle diameter is reduced, then these drawbacks are automatically ameliorated. Also, if the proportion of solder powder with a small particle diameter is small, the solder powder as a whole is not as susceptible to oxidation, so another advantage is a longer life for the solder paste.
The average particle diameter of the solder powder as a whole should be suitably determined as dictated by the thickness of the mask, the diameter of the openings formed therein, and so on, but is 5 to 20 xcexcm, for example.
One or more elements selected from the group consisting of tin, lead, silver, antimony, bismuth, copper, indium, and zinc can be used favorably as the solder component that makes up the solder powder, for example. More specifically, 63%Sn-Pb (melting point: 183xc2x0 C.), Sn-3.5%Ag (melting point: 221xc2x0 C.), 5% Sn-Pb (melting point: 315xc2x0 C.), and the like can be used to advantage.
Meanwhile, the flux vehicle can contain rosin, an activator, and a solvent.
The primary role of the rosin is to increase the adhesion of the solder paste. A variety of known rosins can be used, examples of which include polymerized rosin, hydrogenated rosin, and esterified rosin.
The primary role of the activator is to remove the oxidation film formed on the surface of the electrode pads or on the surface of the individual solder powder particles when the solder paste is heat treated. An organic acid or an organic amine can be used, for example, as this activator. This is because, in most cases, an organic acid has carboxyl groups in the skeleton of the molecular structure, while an organic amine has amino groups in the skeleton of the molecular structure, so both are able to remove the oxidation film from the solder powder surface and the electrode surface in the solder paste.
At least one type of organic acid or organic amine selected from the group consisting of sebacic acid, succinic acid, adipic acid, glutaric acid, triethanolamine, and monoethanolamine is used as the activator. For the action of the activator to be maximized, it is preferable to use one that decomposes or vaporizes near the melting point of the solder. Meanwhile, at temperatures below the melting point of the solder, the activator in the solder paste must be uniformly dispersed in the paste in order for its oxidation film removal effect to be maximized, so the use of one that is miscible with the solvent or rosin is preferred. Accordingly, when an Sn-Ag-based solder is used, for instance, the use of sebacic acid (decomposition temperature: 230 to 290xc2x0 C.), succinic acid (decomposition temperature: 200 to 250xc2x0 C.), adipic acid (decomposition temperature: 230 to 280xc2x0 C.) or the like is preferred.
The amount of activator contained in the solder paste is 0.1 to 2 wt %, for example. If the activator content is too high, it will lead to elevated viscosity of the solder paste, the fluidity of the solder paste will suffer, and it will be difficult to fill the openings in the mask. On the other hand, if the activator content is too low, the oxidation film cannot be sufficiently removed from the solder powder, etc.
The primary role of the solvent is to adjust the viscosity of the solder paste, which is adjusted to between 100 and 400 Paxc2x7s, for example. If the viscosity of the solder paste is lower than 100 Paxc2x7s, when the openings are filled with the solder paste, the resin part (rosin) will be pushed out of the openings, and the wettability of the solder will be impaired. On the other hand, if the viscosity of the solder paste is over 400 Paxc2x7s, it will be difficult for the solder paste to flow into the openings.
It is preferable for the solvent to comprise a combination of a first solvent having a boiling point lower than the melting point of the solder powder, and a second solvent having a boiling point higher than the melting point of the solder powder.
With such a combination, when the solder paste is heated, the first solvent will vaporize before the solder powder melts, and the second solvent will vaporize after the solder powder has begun to melt. The result of this is that the first and second solvents ensure that there is enough solvent to adjust the viscosity of the solder paste, while allowing a reduction in the amount of solvent that vaporizes after the solder powder has begun to melt. Consequently, less heat is robbed from the solder as heat of vaporization during the vaporization of the solvent, and there is less drop in solder temperature during heating, which minimizes the problem of unmelted solder powder remaining behind.
Meanwhile, once the solder begins to melt, the second solvent begins to vaporize, but a specific amount of the second solvent remains for a certain length of time thereafter. A specific amount of solvent needs to remain when the solder is melted in order to maintain the fluidity of the rosin or other resin component and to keep the activator from being taken away along with the vaporization of the solvent, and thereby allow the activator to fine its way into all the parts of the solder and act most effectively. This is the role of the second solvent.
Thus, combining a first solvent with a second solvent ensures that the openings in the mask will be properly filled with solder paste of the desired viscosity, and the activator effectively acts to cause the solder powder particles to fuse together, allowing solder bumps with little variance to be formed. As a result, it is possible to form solder bumps more precisely, and it is possible to form solder bumps accurately at a fine pitch on electrode pads provided at a fine pitch, as is the case with semiconductor elements and so forth.
For this effect to be achieved in the best way, the first solvent is preferably one that has a boiling point 5 to 50xc2x0 C. lower than the melting point of the solder powder, and the second solvent is preferably one that has a boiling point 5 to 50xc2x0 C. higher than the melting point of the solder powder. In other words, if the boiling point of the first solvent is too low, the first solvent may evaporate at room temperature, causing the viscosity of the solder paste to rise, but if the boiling point is too high, it will be close to the melting point of the solder powder, making it impossible to sufficiently reduce the amount of heat robbed by the vaporization of the first solvent when the solder powder melts. Meanwhile, if the boiling point of the second solvent is too high, it will be impossible to sufficiently vaporize the second solvent in the course of heating the solder paste, but if this boiling point is too low, it will be close to the melting point of the solder powder, the activator will be taken away as the second solvent vaporizes, and the activator will not adequately fulfill its function.
The types of first and second solvents used are determined by the melting point of the solder, and mainly by the composition of the solder powder. Table 1 below gives typical solder powder compositions and suitable compositions for the first and second solvents.
It is preferable for the first and second solvents each to be contained in an amount of 2 to 6 wt % in the solder paste in order for them to fulfill their above-mentioned roles as the first and second solvents.
A thixotropic agent may be admixed to the flux vehicle in order to impart shape retention properties to the solder paste. Any of a variety of known thixotropic agents can be used, such as hardened castor oil or hydroxystearic acid.
All of the components used as constituent components of the solder paste preferably either contain no halogen elements or alkali metal elements, or contain these in extremely small amounts. This is because if halogen elements or alkali metal elements remain after the solder bumps have been formed, corrosion can cause degradation of the semiconductor element, or migration can cause shorting between the electrodes. It is particularly favorable for the halogen element and alkali metal element content in the flux vehicle to be no more than 100 ppm.
In a preferred embodiment, the mask is provided over the substrate through the steps of forming a first cover layer over the substrate, forming a second cover layer over this first cover layer, and forming the plurality of openings in the first cover layer and the second cover layer by exposing these to light in a pattern corresponding to the plurality of electrode pads and developing with an etchant, and the first cover layer is formed from a material that will be dissolved by the etchant used to develop the second cover layer, with the etching of the first cover layer being carried out simultaneously with the developing of the second cover layer.
With this bump formation method, just the portion of the second cover layer corresponding to the electrode pads is selectively removed in the developing that follows optical exposure, and first openings that constitute the above-mentioned openings are formed in the second cover layer. Meanwhile, because the first cover layer is formed from a material that will be dissolved by the etchant used to develop the second cover layer, the first cover layer is also etched at the same time by the above-mentioned etchant. Here, since the first cover layer is formed underneath the second cover layer, the second cover layer in which the first openings are formed functions as an etching mask for the first cover layer. Therefore, just the portion of the first cover layer corresponding to the electrode pads and corresponding to the first openings formed in the second cover layer is selectively removed to form second openings that constitute the above-mentioned openings. Thus, with the above bump formation method, there is no need for the first cover layer and the second cover layer to be etched separately in the formation of the openings, which is advantageous in that the work is more efficient.
Unless otherwise specified, the term xe2x80x9coptical exposurexe2x80x9d encompasses irradiation with X-rays, an electron beam, or the like.
The material used to form the first cover layer should be one that will be dissolved by the etchant used in the developing of the second cover layer, and may be suitably selected as dictated by the type of etchant being used.
The material used to form the second cover layer is a macromolecular compound that is photosensitive, or a mixture of a photosensitive compound and another compound, for example, but can be either a negative type in which the portion irradiated with light is cured, or a positive type in which the portion irradiated with light is decomposed. The meaning of the word xe2x80x9cphotosensitivexe2x80x9d here is not limited to the property of undergoing curing (reaction) or decomposition (reaction) when irradiated with light, and also encompasses the property of undergoing curing (reaction) or decomposition (reaction) when irradiated with an electron beam, X-rays, or the like.
Examples of materials with which a negative type cover layer can be formed include polymerizable vinyl group-containing vinyl esters, styrene, acrylic esters, methacrylic esters, and other such monomers, as well as oligomers of these monomers, unsaturated polyester resins, and urea acrylates, and acrylic monomers and oligomers having polymerizable unsaturated double bonds. Naturally, a negative type cover layer may be formed from just a photosensitive compound, or it may be formed from a mixture of a photosensitive compound and another compound, such as an acrylic-, epoxy-, or imide-based macromolecular compound.
Examples of materials with which a positive type cover layer can be formed include macromolecular compounds having ether bonds that readily undergo photolysis (such as polyethylene oxide, cellulose, and polyacetal), as well as polyethylene and other macromolecular compounds that readily produce radicals under optical irradiation, and mixtures of low molecular weight compounds that are decomposed by optical irradiation, such as diazo compounds, with another compound.
In a preferred embodiment, the first cover layer is formed from a material containing a macromolecule that is water-soluble or readily dissolves in an alkaline aqueous solution.
With this bump formation method, the second cover layer can be removed at the same time if at least the first cover layer is dissolved by water or an aqueous solution such as an alkaline aqueous solution. Specifically, if the second cover layer is formed from a material containing a macromolecule that is water-soluble or readily dissolves in an alkaline aqueous solution just as is the first cover layer, then the second cover layer can also be dissolved away at the same time by water or an aqueous solution such as an alkaline aqueous solution. On the other hand, if the second cover layer is formed from a material that contains as its main component a macromolecule has poor solubility in water or in an alkaline aqueous solution, then just the first cover layer can be dissolved. Since the first cover layer is formed underneath the second cover layer, once the first cover layer is dissolved, the second cover layer will no longer be attached to the substrate. In this state, the second cover layer can be easily removed in the form of a film, even if the second cover layer itself is not dissolved. Since there is no need to dissolve the second cover layer in this case, an advantage is that less water or aqueous solution such as an alkaline aqueous solution is used. Therefore, in this respect it is preferable to form the second cover layer from a material whose main component is a macromolecular that has poor solubility in water or in an alkaline aqueous solution.
When the second cover layer has poor solubility in water or in an alkaline aqueous solution, it is preferable for the second cover layer to contain a macromolecule based on an acrylic (such as an acrylic ester), an epoxy (such as a bisphenol A type), or an imide (such as a bismaleimide type of polyimide). Naturally, a combination of these macromolecules may also be used.
The macromolecule that is water-soluble or readily dissolves in an alkaline aqueous solution and is contained in the first cover layer can be a natural macromolecule such as animal-derived gelatin or vegetable-derived starch, a semi-synthetic macromolecule such as a starch derivative or a cellulose derivative, as well as various other macromolecules. Homopolymers (straight polymers) and copolymers can both be used as synthetic macromolecules. Examples of homopolymers include polyvinyl alcohol, polyvinyl pyrrolidone, and other vinyl-based polymers, polyacrylamide, polyacrylic acid, and other acrylic polymers, and polyethylene oxide. Examples of copolymers include random copolymers such as a partially saponified polyvinyl acetate, block copolymers such as poly(styrene-ethylene oxide), and graft copolymers such as poly(ethylene-vinyl alcohol)-g-(ethylene oxide).
In a preferred embodiment, the plurality of electrode pads are divided into a plurality of groups, and the mask is formed through the steps of forming a cover layer so as to cover the plurality of electrode pads, and forming the plurality of openings in this cover layer in a pattern corresponding to the plurality of electrode pads, with the volume of these openings being different for each group.
With this bump formation method, the amount of solder paste filling the various openings is different for each group of electrode pads. Accordingly, it is possible for the bums formed on the electrode pads to be different sizes for each group.
For example, if first openings formed corresponding to the various members of a first electrode pad group out of a plurality of groups are larger in volume than second openings formed corresponding to the various members of a second electrode pad group out of a plurality of groups, then the amount of solder paste filling the first openings will be greater than the amount of solder paste filling the second openings. Accordingly, when the bumps are finally formed, those bumps formed on the first electrode pads will be larger than the bumps formed on the second electrode pads.
In a preferred embodiment, the plurality of electrode pads are divided into a group comprising a plurality of first electrode pads and a group comprising a plurality of second electrode pads, each of the first electrode pads being formed in a surface area smaller than each of the second electrode pads, and the plurality of openings include a plurality of first openings formed in a pattern corresponding to the plurality of first electrode pads, and a plurality of second openings each smaller in volume than each of the first openings and formed in a pattern corresponding to the plurality of second electrode pads.
With this bump formation method, if the thickness of the cover layer is uniform under conditions in which no molten solder is in contact with the inner walls of the openings when the solder is melted, for instance, then the solder bumps will be taller when formed in openings of greater volume. In other words, the larger is an opening, the greater is the amount of solder paste that fills it, so the bump formed on that electrode pad will be taller. If an electrode pad is small, then there will be less contact surface area between the electrode pad and the bump, and the bump will be closer to spherical in shape, so the height of the bumps can be varied in this respect as well.
Thus, if openings of different size are formed in the cover layer, and the surface area of the electrode pads is also different, then a plurality of bump groups of varying distance from the electrode pads to the bump tops can be formed simultaneously and in the same step.
The above description is of an example in which two types of bump with different heights are formed, but of course the present invention can also be applied to when three or more types of bump of different heights are formed. For instance, in addition to first and second openings of different open volume, third openings with yet a different volume may be provided, and of course fourth or further openings may also be provided.
The cover layer is formed, for example, by coating with a molten resin, or laying down a resin film. However, forming the cover layer by laying down a resin film is advantageous, not only because the step of forming the cover layer is easier, but also because it is possible to form a cover layer of uniform thickness with ease.
The cover layer can be made up of a highly insulating resin based on a resin such as polymethyl methacrylate, polyacrylate, or polymethyl isopropenyl ketone, and is preferably made up of a photosensitive material containing a photopolymerizable monomer such as a polyfunctional acrylate.
In a preferred embodiment, wherein the plurality of electrode pads include a plurality of first electrode pads and a plurality of second electrode pads, and the plurality of openings include a plurality of first openings, a plurality of second openings, and a plurality of third openings, and the mask is formed through the steps of forming a first cover layer by covering the plurality of first electrode pads and exposing the plurality of second electrode pads, forming the plurality of first openings in this first cover layer in a pattern corresponding to the plurality of first electrode pads, forming a second cover layer so as to cover the first cover layer and the plurality of second electrode pads, forming the plurality of second openings in the second cover layer in a pattern corresponding to the plurality of second electrode pads, and forming the plurality of third openings in a pattern corresponding to the plurality of first openings.
With this bump formation method, a mask is constituted by the first cover layer and the second cover layer in the region where the first electrode pads are formed, and the mask is constituted by only the second cover layer in the region where the second electrode pads are formed. The first and third openings are provided over the first electrode pads, and the second openings are formed over the second electrode pads. Since the second and third openings are both formed in the second cover layer, if the thickness of the second cover layer is uniform, the depth of these openings will be the same. Accordingly, the openings formed over the first electrode pads are deeper than those formed over the second electrode pads by the depth of the first openings (the thickness of the first cover layer). Therefore, when the above-mentioned mask is used, the amount of solder paste resting on the first electrode pads will be greater than that resting on the second electrode pads, and the solder bumps formed thereon will also be taller. As a result, with this bump formation method, it is possible to form a plurality of bump groups with significantly varying distances from the substrate surface to the bump tops.
The third openings are preferably formed larger than the second openings. If they are, then the amount of solder paste resting on the first electrode pads will be larger than the amount of solder paste resting on the second electrode pads, and as a result the height of the bumps formed on the first electrode pads can be significantly different from the height of the bumps formed on the second electrode pads.
In a preferred embodiment, the third openings are formed with a larger open surface area than the first openings, and there is further included a step of selectively removing just the second cover layer, with the first cover layer left on the substrate.
With this bump formation method, the first cover layer, which has first openings with a smaller open surface area than the third openings, remains after the bumps have been formed, so bumps in which a spherical portion, example, protrudes from the surface of the first cover layer are formed over the first electrode pads such that they are raised up to the remaining first cover layer. Meanwhile, spherical bumps, for example, are formed directly on the second electrode pads. Accordingly, a height difference can be achieved between the bumps over the first electrode pads and the bumps over the second electrode pads.
The first and second cover layers are formed, for example, by coating with a molten resin, or laying down a resin film. However, forming the cover layers by laying down a resin film is advantageous, not only because the step of forming the cover layer is easier, but also because it is possible to form a cover layer of uniform thickness with ease.
The first cover layer can be made up of a highly insulating resin based on a resin such as epoxyacrylate, epoxy, and polyimide.
The second cover layer can be made up of a highly insulating resin based on a resin such as polymethyl methacrylate, polyacrylate, or polymethyl isopropenyl ketone, and is preferably made up of a photosensitive material containing a photopolymerizable monomer such as a polyfunctional acrylate. The first cover layer must be a material that exhibits chemical properties different from those of the second resin film so that it will not be etched by the etchant when the second and third openings are formed in the second resin film. For example, the first resin film can be made up of a material such as epoxyacrylate, epoxy, and polyimide. If it is, not only will the step of forming the resin film be easier, but it will also be possible to form a resin film of uniform thickness with ease.
In a preferred embodiment, the filling of the openings with solder paste is carried out through the steps of holding the substrate on a substrate support, providing squeegeeing helper means for lessening the difference between the height position of the mask and the height position of the periphery of the substrate, readying solder paste on the mask or the squeegeeing helper means, and moving a squeegee to push the solder paste down into the openings.
The filling of the openings with the solder paste need only comprise the various steps listed above, and does not necessarily have to follow the above order. For instance, the openings may be formed after the formation of the cover layers on the substrate, and the openings then with the solder paste while the substrate is held on a substrate support.
With this bump formation method, the provision of the squeegeeing helper means lessens the difference between the height position of the mask and the height position of the periphery of the substrate. Accordingly, the squeegee can be moved not only over the cover layer, but also over the squeegeeing helper means. In other words, not only the solder paste on the substrate, but also the solder paste on the squeegeeing helper means can be moved at the same time and used to fill in the openings. This means that the various openings can be filled with solder paste easily and reliably even when the bumps are being formed on a substrate with uneven width dimensions (such as a silicon wafer).
As shown in FIG. 18a, when bumps were formed on electrode pads 15a of a disk-shaped substrate 15 such as a silicon wafer, the following problems were encountered in the filling of the openings 16a in a mask 16 with a solder paste P. The filling of the openings 16a with the solder paste P was carried out by readying the solder paste P along a specific edge 16A of the mask 16 and moving a squeegee S to the edge 16B on the opposite side. Here, if we look at the movement path of the squeegee S, we see that, as shown in FIG. 18b, the size of the substrate 15 (mask 16) in the direction perpendicular to the movement direction of the squeegee S increases along with the movement of the squeegee S at first, but decreases after passing the widest portion. Therefore, if the solder paste P is readied near the starting point of the squeegee S, the solder paste P can only be moved in a width roughly corresponding to the length of the readied solder paste P. This makes it difficult to properly fill the openings 16axe2x80x2 formed along the edge of the above-mentioned widest portion with a sufficient amount of the solder paste Pxe2x80x2. Also, if the openings 16a are formed right up to the edge of the mask 16, then when the squeegee S is moved up to the opposite edge 16B of the mask 16 in order to fill these openings 16a with the solder paste, the solder paste P will end up being scraped off the mask 16. Accordingly, this solder paste P cannot be moved back to the starting portion of the squeegee S by using the squeegee S, which is a problem in that the solder paste is not utilized effectively.
In contrast, with the above-mentioned solder bump formation method, the periphery of the substrate is surrounded by the squeegeeing helper means, so if the squeegeeing helper means is taken into account, the size in the direction perpendicular to the movement direction of the squeegee can be made larger than the widest section of the substrate, and the distance that the squeegee moves can also be made larger than the substrate.
Therefore, if solder paste is readied on the cover layer or on the squeegeeing helper means so as to correspond to, or be longer than, the widest section of the substrate, and this is moved by the squeegee, then even those openings formed at the widest section of the substrate, or in the vicinity thereof, can be properly filled with solder paste. Also, if the distance the squeegee moves during filling can be made larger, then the solder paste can be moved back on the squeegeeing helper means even when the squeegee has reached the edge of the substrate, so this solder paste can be reused.
For this effect to be achieved in the best way, it is preferable for the squeegeeing helper means and the cover layer to be in the same or approximately the same plane, but as long as the movement of the squeegee is not hindered, there may be some difference in the height of these.
The squeegeeing helper means is provided by forming a resin layer so as to surround the periphery of the substrate, or by disposing a plate or the like having an opening corresponding to the shape of the substrate so as to surround the periphery of the substrate. The squeegeeing helper means may have an opening through which all of the openings provided to the cover layer can be exposed, and as long as the movement of the squeegee is not hindered, the squeegeeing helper means may be provided so that it covers the cover layer, and the surface of the squeegeeing helper means is higher than the surface of the cover layer. Naturally, to the extent that the object of the present invention can still be achieved, the squeegeeing helper means does not necessarily have to be provided so as to surround the entire periphery of the substrate, and need only be provided to the required region of the substrate periphery. Also, the squeegeeing helper means does not necessarily have to be provided as a single integrated member or element, and a plurality of members or elements may be combined to make up the squeegeeing helper means.
However, the resin layer cannot be reused after it has been removed following bump formation, but a plate can be used over and over, so in this respect it is preferable to provide the squeegeeing helper means by disposing a plate. Also, if the squeegeeing helper means comprises a plate, any excess solder paste that did not fill the openings in the cover layer can be moved back on the squeegeeing helper means, and this solder paste that has been moved back to the squeegeeing helper means can be reused in the formation of bumps on the next substrate.
When the squeegeeing helper means is formed from a resin layer, it is preferable to form the resin layer from a material that dissolves in the same etchant as the cover layer, for example, in order to remove the resin layer at the same time in the removal of the cover layer.
Meanwhile, it is preferable for the substrate support to have a recess capable of accommodating at least part of the substrate. If it does, then movement of the substrate with respect to the substrate support can be restricted even when the squeegee is moved over the substrate in order to fill the openings with the solder paste. Since the substrate can be set in place on the substrate support just by placing the substrate in the recess, and does not need to be fixed to the substrate support with an adhesive or the like, this is advantageous in terms of both cost and work efficiency.
The bump formation method provided by the second aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of electrode pads, comprising the steps of forming a first cover layer over the substrate, forming a second cover layer over the first cover layer, forming a plurality of openings corresponding to the plurality of electrode pads in the first cover layer and the second cover layer by exposing these to light and developing with an etchant, filling each of the openings with metal, and heating the metal to integrate it with the electrode pads, wherein the first cover layer is formed from a material that will be dissolved by the etchant used to develop the second cover layer, and the first cover layer is etched to form the plurality of openings simultaneously with the developing of the second cover layer.
With this bump formation method, at the same time that the first openings constituting the above-mentioned openings are formed in the second cover layer by the etching of the second cover layer, the second openings constituting the above-mentioned openings are also formed in the first cover layer using the second cover layer as a mask. Therefore, openings can be formed substantially in the second cover layer by etching without separately etching the cover layers in the formation of the openings.
In a preferred embodiment, the first cover layer is formed from a material containing a macromolecule that is water-soluble or readily dissolves in an alkaline aqueous solution.
With this bump formation method, the second cover layer dissolves at the same time if at least the first cover layer is dissolved by water or an aqueous solution such as an alkaline aqueous solution, or the second cover layer is separated from the substrate, so the entire mask can be removed.
The bump formation method provided by the third aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of electrode pads divided into a plurality of groups, comprising the steps of forming a mask having a plurality of openings corresponding to the plurality of electrode pads such that the size is different for each group, filling the openings with solder paste, forming bumps from the solder paste by heat treatment, and removing the cover layer from the substrate.
With this bump formation method, the amount of solder paste filling the various openings is different for each group of electrode pads. Accordingly, it is possible for the bums formed on the electrode pads to be different sizes, and for the height of the bumps to be different for each group.
One way to make the height of the bumps different for each group is to divide the plurality of into a group of first electrode pad groups and a group of second electrode pads with a larger surface area, and make the volume of the first openings formed in a pattern corresponding to the first electrode pads smaller than the volume of the second openings corresponding to the second electrode pads.
The larger the amount of solder paste on the electrode pads, the larger the bumps will be formed, and if the thickness of the cover layer is uniform and the molten solder does not touch the inner walls of the openings, then the smaller is the surface area of the electrode pads, the taller the bumps will be. Accordingly, if the relationship between the openings and the electrode pads is as above, then bumps of different heights can be formed more reliably.
The cover layer is formed, for example, by coating with a molten resin, or laying down a resin film. Examples of the component that makes up this cover layer include polymethyl methacrylate, polyacrylate, or polymethyl isopropenyl ketone. These components may be used singly or in combinations of two or more types.
The bump formation method provided by the fourth aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of first electrode pads and a plurality of second electrode pads, comprising the steps of forming a first cover layer in a state in which the plurality of first electrode pads are covered and the plurality of second electrode pads are exposed, forming a plurality of first openings in the first cover layer in a pattern corresponding to the plurality of first electrode pads, forming a second cover layer so as to cover the first cover layer and the plurality of second electrode pads, forming a plurality of second openings in the second cover layer in a pattern corresponding to the plurality of second electrode pads, and forming a plurality of third openings in a pattern corresponding to the plurality of first openings, filling the first openings, second openings, and third openings with solder paste, forming bumps from the solder paste by heat treatment, and removing the second cover layer.
With this bump formation method, the thickness of the mask is different in the region in which the first electrode pads are formed and in the region in which the second electrode pads are formed. Therefore, the height is different between the bumps formed on the first electrode pads and the bumps formed on the second electrode pads.
It is preferable for the surface area of the third openings to be made larger than that of the second openings in order to make the amounts of solder paste filling the insides of the second and third openings markedly different and to achieve a good difference in height between the bumps on the first electrode pads and those on the second electrode pads.
Also, the third openings may be formed with a larger open surface area than the first openings, and just the second cover layer may be selectively removed, with the first cover layer left on the substrate.
If this is done, the first cover layer remains after the bumps have been formed, so bumps in which a spherical portion, example, protrudes from the surface of the first cover layer are formed over the first electrode pads such that they are raised up to the remaining first cover layer. Meanwhile, spherical bumps, for example, are formed directly on the second electrode pads. Accordingly, a good difference in height can be obtained between the bumps over the first electrode pads and the bumps over the second electrode pads.
The first and second cover layers are formed, for example, by coating the substrate with a molten resin, or laying a resin film over the substrate.
The first cover layer can be made up of a highly insulating resin based on a resin such as epoxyacrylate, epoxy, and polyimide.
The second cover layer can be made up of a highly insulating resin based on a resin such as polymethyl methacrylate, polyacrylate, or polymethyl isopropenyl ketone.
The bump formation method provided by the fifth aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of electrode pads, comprising the steps of holding the substrate on a substrate support, forming a cover layer so as to cover at least the substrate, forming a plurality of openings in the cover layer in a pattern corresponding to the plurality of electrode pads, providing squeegeeing helper means for lessening the difference between the height position of the cover layer on the substrate and the height position of the periphery of the substrate, readying a metal paste (including solder paste) or metal powder on the cover layer or the squeegeeing helper means, moving a squeegee to push the metal paste or metal powder down into the openings, heating, melting, and, solidifying the metal paste or metal powder to integrate it on the electrode pads, and taking away the squeegeeing helper means.
This bump formation method need only comprise the various steps listed above, and does not necessarily have to follow the above order. For instance, the openings may be formed after the formation of the cover layer on the substrate, and the openings then filled with the solder paste while the substrate is held on a substrate support.
With this bump formation method, the provision of the squeegeeing helper means lessens the difference between the height position of the mask and the height position of the periphery of the substrate. Accordingly, the metal powder or metal paste can be moved not only over the cover layer, but also by utilizing the squeegeeing helper means, allowing the individual openings to be reliably filled with metal powder, etc., even when the bumps are formed on a substrate whose width is not even.
For this effect to be achieved in the best way, it is preferable for the squeegeeing helper means and the cover layer to be in the same or approximately the same plane, but as long as the movement of the squeegee is not hindered, there may be some difference in the height of these.
The squeegeeing helper means is provided by forming a resin layer so as to surround the periphery of the substrate, or by disposing a plate or the like having an opening corresponding to the shape of the substrate so as to surround the periphery of the substrate. The squeegeeing helper means may have an opening through which all of the openings provided to the cover layer can be exposed, and as long as the movement of the squeegee is not hindered, the squeegeeing helper means may be provided so that it covers the cover layer, and the surface of the squeegeeing helper means is higher than the surface of the cover layer.
Meanwhile, it is preferable for the substrate support to have a recess capable of accommodating at least part of the substrate. If it does, then movement of the substrate with respect to the substrate support can be restricted, allowing the squeegee to move more smoothly.
In all of the first to fifth aspects of the present invention discussed above, it is preferable if there is further provided a step of applying flux to the bumps formed from heat treated solder paste, and performing a heat treatment again to adjust the shape of the bumps.
A flux containing Polypale and hexylene glycol is used, for example.
In all of the first to fifth aspects of the present invention discussed above, it is preferable if the open surface area of the openings is no more than 25 times the surface area of the corresponding electrode pads. If it is, the molten solder can be gathered more reliably on the electrode pads when the solder is melted, allowing the solder to be formed in good spherical shapes.
The sixth aspect of the present invention provides an electronic component, comprising a substrate, a plurality of first electrode pads and a plurality of second electrode pads formed on the same surface of this substrate, a plurality of first bumps formed in a pattern corresponding to the plurality of first electrode pads, and a plurality of second bumps formed in a pattern corresponding to the plurality of second electrode pads, wherein the surface area of each of the first electrode pads is smaller than the surface area of each of the second electrode pads, and the top of each of the first bumps is located higher than the top of each of the second bumps.
The seventh aspect of the present invention provides an electronic component, comprising a substrate, a plurality of first electrode pads and a plurality of second electrode pads formed on the same surface of this substrate, a cover layer formed in the region of the substrate where the plurality of first electrode pads are formed and having a plurality of openings corresponding to the plurality of first electrode pads, a plurality of first bumps provided in a pattern corresponding to the plurality of first electrode pads, with spherical portions protruding from the cover layer, and a plurality of second bumps provided in a pattern corresponding to the plurality of second electrode pads, with spherical portions formed directly on the corresponding second electrode pads, wherein the top of each of the first bumps is located higher than the top of each of the second bumps.
The cover layer can be made from a highly insulating resin based on an epoxyacrylate, epoxy, polyimide, or other such resin.
A preferred embodiment of the electronic components discussed in the above-mentioned sixth and seventh aspects of the present invention is an electronic component further comprising a mounting object, wherein this mounting object is placed on the substrate with the plurality of second bumps therebetween, and the top of each of the first bumps is located at a height of at least 1.2 times the height location of the top of the mounting object.
With this structure, an additional mounting object can be placed on the substrate via the first bumps in a state in which the original mounting object is interposed between the additional mounting object and the substrate, or the substrate can be mounted on another substrate via the first bumps. Employing this structure affords higher mounting efficiency in the mounting of an electronic component on the substrate, and allows for a more compact electronic component consisting of a plurality of semiconductor chips or the like.
An electronic component having two kinds of bumps of different size was described in the sixth and seventh aspects of the present invention, while a method for forming two kinds of bumps of different size as needed was described in the first through fourth aspects of the present invention. However, when three or more kinds of bump of different size are to be formed, the present invention can be applied whenever any two bumps are of different size, and is not necessarily limited to when two kinds of bump of different size are formed.
The eighth aspect of the present invention provides a solder paste containing a solder powder and a solvent, wherein the solvent contains a first solvent having a boiling point lower than the melting point of the solder powder, and a second solvent having a boiling point higher than the melting point of the solder powder.
This solder paste can be used favorably in the formation of second bumps. In a preferred embodiment, the same solder paste as that discussed in the eighth aspect of the present invention can also be used in the first aspect of the present invention. Therefore, when the solder paste of the eighth aspect of the present invention is used to form second bumps, the same effects will be realized as when the solder paste of the first aspect of the present invention was used.
Specifically, since the first solvent has already vaporized in the melting of the solder, less heat is robbed from the solder through heat of vaporization of the solvent after the solder begins to melt, the effect being an amelioration of the problem of unmelted solder powder being left behind. Because a specific amount of the second solvent remains even after the solder has melted, the fluidity of the rosin or other resin component is maintained, the activator is not carried away as the solvent is vaporized, and the activator is able to get into all parts of the solder, allowing it to act more effectively. As a result, it is possible to form solder bumps with a good spherical shape and no variance in size.
For this effect to be obtained in an even better way, it is preferable for the first solvent to have a melting point 5 to 50xc2x0 C. lower than the melting point of the solder powder, and for the second solvent to have a melting point 5 to 50xc2x0 C. higher than the melting point of the solder powder. For the same reason, it is preferable for the first solvent to be contained in the solder paste in an amount of 2 to 6 wt %, and the second solvent in an amount of 2 to 6 wt %.
The types of first and second solvent to be used will be determined by the type (melting point) of the solder powder being used, and in the eighth aspect of the present invention, it is again preferable to use those listed as examples for the first aspect of the present invention (see Table 1).